1. Field of the Invention
The present invention relates generally to an image reading apparatus for reading an image of the original, and more particularly to an image reading apparatus for reading in a way that brings a reading unit into a contact with a placing member on which the original is placed.
2. Description of the Related Art
Some of book-oriented image reading apparatuses utilized for, e.g., copying machines, image scanners etc. have hitherto used CCDs and contact image sensors as image sensors.
In the case of using the image sensor, an optical system can be simplified and decreased in weight, and hence the whole apparatus can be downsized.
This type of image sensor, however, has a such a problem that a focal depth is small, and the resolution declines due to a fluctuation of distance between a platen glass and the image sensor.
In this respect, Japanese Patent Application Laid-Open No. 07-193689 copes with the above problem by providing a high-slidability spacer on a surface of the image sensor that faces to the platen glass, moving by pressure the image sensor towards the platen glass and thus keeping constant a gap between the platen glass and the image sensor.
In the case of using the image sensor, there is taken a method of providing an abutting member for restraining a deviation with respect to the focal depth and moving the image sensor in a state where the abutting member is brought into a frictional contact with the platen glass.
According to this method, however, triboelectrification might occur due to a frictional slide between the abutting member and the platen glass.
Then, when electric charges generated are discharged, electrostatic noises enter the image sensor disposed within an image reading unit and an electric circuit, with the result that an image output might be disturbed.
FIG. 5 is a schematic view representing an image defect caused by discharging the electric charges described above. FIG. 5 shows the enlarged view showing herein an image outputted in the case of using an original containing an uniform black image in a front half thereof and a white image in a rear half thereof. As illustrated in FIG. 5, a white spot pattern irregularly appears in the black image area, while a black spot (fog) pattern likewise appears in the white image area.
Further, in the case of copying an image emphasizing a gradation reproducibility such as a photographic image etc by use of a copying machine and so on, a uniformity of density is important. In this case, however, the uniformity of an output image declines due to the discharge noises entering the reading signals.
Japanese Patent Application Laid-Open No. 09-284476 discloses, as a measure against the static electricity caused by the frictional slide between the abutting member and the platen glass, a construction including a static electricity preventing mechanism for preventing the static electricity from being generated in a portion at which a roller for keeping a gap constant abuts on the platen glass.
This construction, to be specific, aims at de-electrifying the abutting portion between the roller and the platen glass in a way that attaches a de-electrifying cloth to an outer peripheral portion of the image sensor or a harness and thus preventing an electrostatic adhesion of dusts to the abutting member.
With the construction described above, however, the platen glass can be de-electrified by making the de-electrifying member abutting thereon. The abutting member itself can not be, however, de-electrified. Further, the de-electrifying cloth is easy to fall and fold down because of being used for a long period of time, and the effect thereof does not continue. Moreover, a conveying resistance increases due to the de-electrifying cloth, and hence a drive load rises, with the result that the apparatus architecture becomes hard to downsize and the electric power consumption gets difficult to decrease.
Further, Japanese Patent Application Laid-Open No. 07-193689 does not particularly deal with an electric resistance value of the abutting member and a de-electrification preventing process, nor does it show any measure against the occurrence of the image noises due to the discharge of the static electricity described above.
Furthermore, U.S. Pat. No. 6,008,485 exemplifies a ball bearing composed of a metallic material as an abutting member but has a problem, wherein the ball bearing is hard to control the gap related to the abutting member, and requires an additive such as an oil for lubricating the drive of the bearing, which is unpreferable in terms of contamination.